A person who is ignorant and generally has no idea about anything.

A right useless fuckwit.

Someone who is useless until you shove your fist up them - Just like Kermit !

A nicer way to say you are an idiot, or a retard.

E.G. "OMG you are such a muppet!" or "Haha you muppet!"

You could use this when talking to friends if you don't want to use something as harsh as idiot.

"Well, it's not quite a mop, it's not quite a puppet, but man... " - Homer Simpson

an affectionate term for someone who's a bit stupid

A Stupid person which has nothing better to do